Cancer stem cells (CSCs) from oral squamous cell carcinomas (OSCCs) are expected to have properties of self-renewal, metastasis, and treatment resistance. Current model systems for studying CSCs are largely in immune compromised and species-mismatched stromal environments, which do not capture all CSC properties. We have developed XactMice in which human OSCCs are transplanted into mice bearing human hematopoietic stem cells (HSC) that reconstitute human immune cells and other HSC-derived human stroma cells. OSCCs in XactMice exhibited pathological and molecular signatures more similar to primary human OSCCs than OSCCs in immune compromised recipient mice. Additionally, we developed genetically engineered mouse models (GEMMs) with spontaneous OSCCs. Using these complementary models, we will study the influence of the microenvironment on CSC properties, and how this alters treatment resistance. First, we will examine if immune system and species-matched stromal cells affect sizes and behaviors of transplanted CSCs, including cancer initiation, metastasis and resistance to radiation, the main therapy for OSCC. Second, we will combine CSCs in co-culture systems with primary cancer associated fibroblasts (CAFs) or CAFs from XactMice from the same patient, and assess if HSC-derived fibroblasts have the same influence as primary CAFs on CSC self-renewal and invasion, and if a small portion of HSC-derived fibroblasts among mouse fibroblasts is sufficient to contribute to CSC behavioral changes. Finally, based on our preliminary data, we will study if inhibiting TGF? signaling hampers CSC properties of cancer initiation, metastasis and radiotherapy resistance, and identify which TGF? signaling components and targets are involved in CSC regulation. Our efforts to transform human cancer models in which human OSCCs grow in immune competent and species-matched stromal cells provide an ideal platform to study the interaction between CSCs and stroma in OSCC. These experiments will advance understanding of CSC biology, tumor- stroma interactions in human cancer, and the prioritization of novel targeted molecules for inhibiting cancer growth and metastasis.